Testing for Huntington’s disease relies on a specific genetic test that counts the number of repeated DNA segments in a single gene. A simple blood draw is all that’s needed for the test itself, but the process around it, particularly for people without symptoms, involves multiple steps designed to support you through what can be a life-changing result.
What the Genetic Test Measures
Huntington’s disease is caused by a mutation in the HTT gene, which provides instructions for making a protein called huntingtin. Within this gene, a short DNA sequence (called a CAG trinucleotide repeat) is repeated a certain number of times. In most people, this segment repeats 10 to 35 times, and the gene functions normally. In people who develop Huntington’s, it repeats 36 or more times, producing an abnormally long version of the huntingtin protein that gradually damages nerve cells in the brain.
The test itself uses a blood sample. A lab extracts your DNA and counts the exact number of CAG repeats in your HTT gene using PCR-based methods, which can detect repeat lengths up to about 115. For very large expansions, a technique called triplet-primed PCR can confirm the presence of an expanded repeat without pinpointing the exact number.
What Your CAG Repeat Number Means
Your result falls into one of four ranges, and each carries a different meaning:
- 10 to 26 repeats: Normal. No risk of developing Huntington’s and no risk of passing it to children.
- 27 to 35 repeats (intermediate): You will not develop Huntington’s yourself, but the repeat can expand when passed to the next generation, putting your children at possible risk.
- 36 to 39 repeats (reduced penetrance): You may or may not develop symptoms during your lifetime. The uncertainty in this range is real, and genetic counseling is especially important here.
- 40 or more repeats (full penetrance): Huntington’s disease will almost always develop at some point. People with 40 to 50 repeats typically develop symptoms in adulthood, while those with more than 60 repeats are more likely to develop the juvenile form.
One important pattern: as the HTT gene passes from parent to child, the number of CAG repeats tends to grow. This means symptoms can appear at a younger age in each successive generation, a phenomenon called anticipation. The expansion is usually larger when the gene is inherited from the father.
Predictive Testing for People Without Symptoms
If you have a parent or close relative with Huntington’s and want to know your own status, you’ll go through what’s called predictive testing. This isn’t a quick in-and-out appointment. Most testing centers follow a multi-visit protocol that includes genetic counseling sessions spread over weeks or months. The process is deliberately paced to give you time to consider what a positive result would mean for your life, your relationships, and your mental health, and to change your mind at any point.
This careful approach exists for good reason. Research has shown that when predictive testing is done with proper counseling and adequate time between steps, it is not associated with negative psychological consequences like increased suicidality. Some studies have found that going through the process can even strengthen relationships, regardless of the result. You’ll typically meet with a genetic counselor, sometimes a psychologist or social worker, and a neurologist before any blood is drawn.
You are never obligated to receive your results once testing has begun. At every stage, you can pause or stop the process entirely.
Clinical Diagnosis When Symptoms Are Present
For someone already showing potential signs of Huntington’s, the diagnostic path looks different. A neurologist will evaluate three core areas: movement, thinking, and behavior. The standard clinical tool is the Unified Huntington’s Disease Rating Scale (UHDRS), which covers 31 items assessing eye movements, muscle rigidity, involuntary movements (chorea), balance, and gait. Cognitive testing includes tasks like naming as many words as possible in a category within a time limit, matching symbols to digits, and a color-word interference test that measures mental flexibility. The behavioral assessment rates the frequency and severity of depression, anxiety, aggression, and psychosis.
Brain imaging with MRI often supports the clinical picture. In Huntington’s, the caudate nucleus and putamen, two structures deep in the brain that help coordinate movement, visibly shrink. This shrinkage causes the fluid-filled spaces near them to widen, a pattern radiologists can spot on routine scans. These imaging changes can sometimes appear before symptoms become obvious.
Even with a classic clinical presentation and characteristic brain imaging, the genetic test confirming an expanded CAG repeat is what makes the diagnosis definitive.
Testing in Children and Juvenile Huntington’s
Juvenile Huntington’s disease, which begins before age 21, accounts for a small percentage of cases and usually involves very large CAG expansions, typically more than 55 repeats, with a median around 60. The longest reported expansion is 250 repeats. Children with juvenile Huntington’s often present differently than adults. Rather than the classic involuntary movements, they may show stiffness, clumsiness, speech difficulties, or declining school performance.
Testing minors who don’t have symptoms is generally discouraged by genetics professionals. The reasoning is that predictive testing removes a child’s future ability to make that choice as an adult. When a child is already showing concerning symptoms, however, genetic testing proceeds as part of the diagnostic workup. DNA analysis is the only test that can make the diagnosis definitive in children, just as in adults. MRI may show caudate atrophy or signal changes in the brain’s deep structures, but these findings alone aren’t conclusive.
Testing Before or During Pregnancy
If you carry an expanded HTT gene and want to prevent passing it on, two main options exist. Preimplantation genetic testing for monogenic disorders (PGT-M) is done during IVF. Embryos are created through standard fertility treatment, and a small group of cells is biopsied from each embryo at the blastocyst stage, typically 5 to 10 cells from the outer layer that becomes the placenta. Only embryos without the expanded repeat are transferred to the uterus.
If you’re already pregnant, chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester can test fetal DNA for the CAG expansion. Some people who undergo PGT-M are also offered CVS or amniocentesis to confirm the result.
A less commonly discussed option is exclusion testing, which can determine whether a pregnancy is at risk without revealing the at-risk parent’s own status. This involves tracking which grandparental chromosome was inherited, rather than directly counting the fetal CAG repeats. It’s a complex process that requires careful genetic counseling.
Insurance, Cost, and Legal Protections
Health insurance often covers genetic testing for Huntington’s when a doctor recommends it, but coverage varies by plan. Contacting your insurer before testing to confirm what’s covered is worth the phone call. Some people choose to pay out of pocket specifically to keep the result off their insurance record.
In the United States, the Genetic Information Nondiscrimination Act (GINA) offers significant but incomplete protection. GINA makes it illegal for employers to use genetic information in hiring, firing, promotions, pay, or job assignments. It also prohibits health insurers from using genetic test results to deny coverage or set premiums. These protections took effect in 2009 and apply broadly to employers with 15 or more employees.
The gap worth knowing about: GINA does not cover life insurance, disability insurance, or long-term care insurance. Insurers in those markets can legally ask about and use genetic test results. This is one reason some genetic counselors discuss the practical implications of testing, including the order in which you apply for various insurance policies, before you proceed.